1. Field of the Invention
The present invention relates generally to a document scanner, and in particular to a method of optimal focusing for a document scanner for obtaining best image of scanning.
2. The Related Art
Document scanners have been widely used to digitalize graphic information from reflective and transmissive original documents. A major factor concerning the performance of the document scanners is the resolution, the fineness of the digitalization of a document. Excellent digitalization of documents can be obtained by upgrading resolution of scanner. However, proper focusing is another factor for the excellent digitalization of a document.
FIG. 1 of the attached drawings shows a conventional document scanner, comprising a casing 2 having a scanning window formed by a rectangular opening to which a light-transmitting document support plate 11 is attached for supporting a document 12 to be scanned in a face down manner. An optical module 2 is movably mounted inside the casing 1 below the document support plate 11. The optical module 2 is driven by an optical module moving mechanism 3 under the control of a control unit 4 for movement in a direction, indicated by arrow I, substantially parallel to the document support plate 11 in order to scan throughout the whole area of the document 12.
The optical module 2 comprises a light source 21 for emitting and projecting a light beam toward the document 12 to be scanned. The document 12 reflects the light as indicated by reference numeral 211. The reflected light 211 is redirected by a plurality of reflectors 22, 23, 24, 25 toward a focusing lens 26 and forms an image of the document on an image sensing device 27, such as a charge coupled device (CCD).
Conventionally, the optical module 2 is calibrated to obtain the best focus for properly forming image on the image sensing device 27. However, the optical module 2 may gradually get out of calibration due to aging and environmental factors, such as exposure to shock and vibration.
Automatic focusing techniques are also employed in the document scanners for obtaining clear images. However, such automatic focusing techniques cannot overcome improper focusing caused by vibration of the scanner itself and worn parts.
In addition, conventionally, in a calibrated system, a document to be scanned is supposed to be on the nominal focal plane of the scanner. It is, however, often that the document is partially out of the nominal focal plane when the document is folded or when the document is not properly positioned on the document support plate. This leads to a partially unclear image of the document.
Thus, it is desired to have a method of automatic focusing for a document scanner in order to obtain clear image of the scanned document